1. Field of the Invention
The present invention relates to a fiber Bragg grating device and, more particularly, to a fiber Bragg grating device that is used as an encoder and a decoder in optical code division multiplexing transmission.
2. Description of Related Art
In recent years, the demand for communication has been increasing at high speed as a result of the popularization of the Internet. Accordingly, high-speed and large-capacity optical networks that employ optical fiber or the like have been developed. In such optical networks, wavelength division multiplexing (WDM) methods are indispensable, there being a particular focus on so-called Dense WDM (DWDM) methods in which wavelength-multiplexing is performed at high densities on the wavelength axis by narrowing wavelength intervals of optical carrier waves allocated to each channel.
In addition to WDM methods or DWDM methods, transmission methods tend to focus on transmission using optical code division multiplexing (OCDM). According to OCDM transmission, transmission is carried out as follows. On the transmission side, optical pulse signals of a plurality of channels are generated in parallel, and the optical pulse signals are modulated (encoded) by means of code that is different for each channel. On the reception side, the modulated optical pulse signals are restored (decoded) to the original parallel optical pulse signals by means of the same code as that used when the optical pulse signals were encoded on the transmission side. Here, an optical pulse signal is an optical pulse train that is rendered by reflecting binary digital electrical pulse signals and that is obtained by subjecting an optical pulse train to optical modulation. Meanwhile, the expression ‘optical pulse train’ is employed when referring to an optical pulse train in which optical pulses are lined up at regular fixed time intervals.
In order to contribute to an understanding of the fiber Bragg grating (FBG) device of the present invention, a representative constitution of the OCDM transmission device will first be described with reference to the block constitution shown in FIG. 1.
The OCDM transmission device comprises a transmission portion 10 and reception portion 30 that are connected by a transmission line 50.
The transmission portion 10 is constituted comprising an optical pulse train generator 12, a modulated signal generator 14, an optical modulator 16, a first optical circulator 18 and an encoder 20.
The optical pulse train generator 12 generates an optical pulse train (shown by an arrow 13 in FIG. 1). The modulated signal generator 14 supplies information that is to be transmitted to the optical modulator 16 as a binary digital electrical pulse signal (shown by an arrow 15 in FIG. 1). The optical modulator 16 subjects the optical pulse train 13 to optical modulation to generate an optical pulse signal (shown by an arrow 17 in FIG. 1) that reflects a binary digital electrical pulse signal 15. The optical pulse signal 17 generated by the optical modulator 16 enters the encoder 20 via the first optical circulator 18. The encoder 20 generates a transmission signal (shown by an arrow 21 in FIG. 1) by encoding the optical pulse signal 17. A transmission signal 21 is sent to the transmission line 50 via the first optical circulator 18 and then sent to the reception portion 30 after being propagated by the transmission line 50.
The reception portion 30 is constituted comprising a second optical circulator 38, decoder 40, and O/E converter 36.
The transmission signal 21 that is transmitted through propagation via the transmission line 50 enters the decoder 40 via the second optical circulator 38. The decoder 40 generates an optical pulse signal (shown by an arrow 31 in FIG. 1) by decoding the transmission signal 21. The optical pulse signal 31 enters the O/E converter 36 via the second optical circulator 38 and is restored as a binary digital electrical pulse signal (shown by arrow 37 in FIG. 1) by the O/E converter 36. That is, the binary digital electrical pulse signal 15, which is information to be transmitted, is propagated via the transmission line 50 as an optical pulse signal 21 and then restored as a result of becoming a binary digital electrical pulse signal 37 in the reception portion 30.
Transmission using OCDM makes it possible to transmit optical pulse signals of a multiplicity of channels at the same time and the same wavelength. Further, transmission using OCDM is a method that uses the same code as a key on the transmission side and reception side, whereby highly stable or safe transmission is obtained.
Phase code system OCDM that employs the phase of light as code is known as OCDM encoding means (see, for example, DOCUMENT 1: P. Petropoulos et al. “Demonstration of a 64-chip OCDMA System Using Superstructured Fiber Gratings and Time-Gating Detection”, IEEE Photonic Technology Letters, Vol. 13, No. 11, November 2001, pp. 1239–1241). More specifically, an FBG device comprising a superstructured fiber Bragg grating (SSFBG) is used as the encoder 20 and decoder 40. The SSFBG comprises a plurality of fiber Bragg gratings (known as ‘FBG units’ hereinafter) with the same constitution in the core of the optical fiber.
With an SSFBG, it can be assumed that the phase difference (known as a ‘relative phase difference’ hereinafter) of the reflected light in adjacent FBG units is 0 or π as a result of establishing the interval between adjacent FBG units. For example, when encoding is performed by means of a 15-bit code array ‘0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1’, the phase of reflected light in the first to fifteenth FBG units is set as ‘0, 0, 0, π, π, π, π, 0, π, 0, π, π, 0, 0, π’ by establishing the interval for adjacent FBG units. Further, in the case of decoding, the arrangement of FBG units in SSFBG is the same as the arrangement for encoding but the light input and output terminals are reversed with respect to the encoder.
In the OCDM encoder and decoder of the conventional example above, the code that is employed in encoding and decoding, that is, the interval between adjacent FBG units is fixed. As a result, to change the code, a set of the encoder and decoder showed be replaced by a new one set. Therefore, as means for converting or changing the code constituting the OCDM encoder and decoder, a phase encoder that regulates the phase shift amount and optionally sets the code by causing a plurality of tungsten wires to touch the SSFBG at fixed intervals and adjusting the interval between adjacent FBG units through localized heating using respective tungsten wires has been tested (see, for example, DOCUMENT 2: M. R. Mokhtar et al., “Reconfigurable Multilevel Phase-Shift Keying Encoder-Decoder for All-Optical Networks”, IEEE Photonics Technology Letters, Vol. 15, No. 3, March 2003, pp. 431 to 433).
However, with the OCDM encoder and decoder appearing in the DOCUMENT 2, when a long time passes after setting the code, the heating area increases as a result of thermal conductivity on the optical fiber. When the heating area increases, the phase shift differs from the desired value, that is, there is the problem that decoding can no longer be performed because the code is different.
Further, when the temperatures of the environment in which the encoder and decoder are installed are different or the environmental temperatures fluctuate, the reflection wavelengths of the encoder and decoder are then different. In OCDM transmission, when there is a wavelength difference of a few picometer (pm) between the reflection wavelengths of the encoder and decoder established with the same code, encoding and decoding cannot be favorably performed.
The present invention was conceived in view of the above problem. An object of the present invention is to provide a fiber Bragg grating device that is used as an OCDM encoder and decoder that is capable of implementing code changes by means of a low-cost and simple constitution, which allows a phase shift to hold the desired value even after a long time has passed and which permits adjustment of reflection wavelengths.